In a typical cellular radio system, wireless user equipment units (UEs) communicate via a radio access network (RAN) with one or more core networks. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network. Alternatively, the wireless user equipment units can be fixed wireless devices, e.g., fixed cellular devices/terminals which are part of a wireless local loop or the like.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station. A cell is a geographical area where radio coverage is provided by the radio equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The radio base stations communicate over the air interface with the user equipment units (UE) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landline or microwave link) to a control node known as a base station controller (BSC) or a radio network controller (RNC). The control node supervises and coordinates various activities of connected plural radio base stations. The control node is typically connected to one or more core networks.
A conventional radio base station in a cellular communications system is generally located in a single location, and the distance between the baseband circuitry and the radio circuitry is relatively short, e.g., on the order of one meter. A distributed radio base station includes the radio equipment control (REC) and the radio equipment (RE). Both parts may be physically separated, (i.e., the RE may be close to the antenna, whereas the REC is located in a conveniently accessible site), or both may be co-located as in a conventional radio base station design. The radio equipment control (REC) performs baseband signal processing, and each radio equipment (RE) converts between baseband and radio frequencies and transmits and receives signals over one or more antennas. Each RE serves a certain geographic area, sector, or cell. Separate, dedicated optical and/or electrical links connect the radio equipment control (REC) to each of the plural remote radio equipment (RE). However, the term link as used here refers to a logical link and is not limited to any particular physical medium. Each link carries digital information downlink from the REC to the RE and digital information uplink from the RE to the REC.
Efforts have been underway to provide a standardized common interface between a REC and one or more REs to enable flexible and efficient product differentiation for radio base stations and independent technology evolution for the RE and REC. One such standard is the Common Public Radio Interface (CPRI), and it defines necessary items for transport, connectivity, and control including user plane data, control and management (C&M) plane transport mechanisms, and synchronization. The CPRI interfaces carries timing information, IQ data samples, and operations and maintenance (O&M) link for communications between the REC and each RE. These three flows are multiplexed in a time division multiplex (TDM) frame structure transferred on the interface based on 8B/10B coding (8 bits of data encoded to 10 bits words). Timing information is communicated to the REs by aligning a hyperframe start in the downlink direction (REC→RE) to a frame start (FS) in the REC. Each RE extracts the hyperframe start and uses it as a recovered frame start. The recovered frame start should be compensated to correct for various delay components associated with the downlink interface. The hyperframe control information includes a known symbol (e.g., in the CPRI the known symbol is a K28.5 symbol) for use in obtaining synchronization between the REC and each RE. The synchronization includes detecting the known symbol to retrieve one or more hyperframe borders.
An important requirement for this kind of distributed base station is accurately measuring and compensating for a transmission time delay associated with the distributed transmission link/internal interface. Typically, a round-trip delay is determined for transmitting a signal from the REC to an RE and returning that signal to the REC. Another key requirement is to transmit data over the radio/air interface with very high timing accuracy. In a distributed base station, this means that two REs coupled to an REC should be very accurately synchronized. In one example scenario, a maximum timing difference between two REs might be on the order of tens of nanoseconds. That maximum timing difference is largely “budgeted” for analog parts, temperature changes, and installation (mis-)calibration. This leaves a timing difference budget for the digital parts only on the order of a few nanoseconds.
When transmitting data over the radio air interface with very high timing accuracy, the REC has a first time maintained by an REC local “air frame” timer or counter when the REC believes that the frame is transmitted over the radio air interface. Similarly, the RE has a second different time maintained by an RE local timer or counter that the RE derives from the frame received from the REC by extracting the K28.5 symbol, hyperframe number, and frame number, as defined in the CPRI specification In other words, the REC digital circuitry and the digital circuitry in each RE operate using different clock domains maintained by respective local timers or counters, i.e., two different clock sources operating at the same frequency but with variable phase. A clock domain is defined as that part of a design which is driven by either a single clock or clocks that have constant phase relationships. Conversely, domains that have clocks with variable phase and time relationships are considered different clock domains. To have the REC and RE local timers being precisely in phase the CPRI interface delay from the REC to the RE must be known. Part of that delay depends on a delay associated with different timing domains in each of the REC and/or RE.
An REC and/or RE itself may also operate using plural clock domains. For example, an REC may couple baseband processing circuitry, including a framer/deframer that uses a first internal clock, to a SERDES that uses a second internal clock. The RE may include a CPRI interface application specific integrated circuit (ASIC) that uses a first internal clock and a radio air interface processing ASIC that uses a second different internal clock. When transferring data from one clock domain to another, a buffer is introduced to synchronize the data. Data from the first clock domain is written to the buffer using a first clock domain timing signal. That data is read from the buffer using a second clock domain timing signal. The time that the data is in the buffer depends on the phase relationship between the two clock domains and is therefore unknown. Because the buffer delay is part of the overall delay between the REC and the RE, it should be measured so that it can be compensated.
Recognizing these problems and challenges, the inventors designed a solution not only for distributed base stations, but also for any situation where a delay is caused by changing between different clock domains using a buffer or the like. Buffer circuitry receives data to be processed by electronic circuitry using a first clock signal associated with a first clock domain. The buffered data is output using a second clock signal associated with a second clock domain. The buffering of the data is associated with a buffering delay. Counting circuitry receives at a start count input a write timing signal associated with the first clock domain and with writing data to the buffer circuitry. The counting circuitry receives at a stop count input a read timing signal associated with the second clock domain and with reading data from the buffer circuitry. The timing signal can be any type of timing signal. Several non-limiting examples include a clock signal, a trigger signal, a strobe signal, etc. A count value accumulated between receiving the write timing signal and the read timing signal corresponds to the buffering delay. Control circuitry performs a control operation based on the count value. For example, the accumulated count value represents a phase difference between the first and second clock signals, and the control circuitry may compensate for the phase difference.
In one non-limiting detailed example embodiment, the first clock domain is associated with first processing circuitry, and the first clock signal is a transmit write clock associated with writing data into transmit buffer circuitry. The second clock domain is associated with second processing circuitry, and the second clock signal is a transmit read clock signal associated with reading data from the transmit buffer circuitry. The first and second processing circuitry can be associated with the same integrated circuit or separate integrated circuits. The counting circuitry includes a transmit counter. The second processing circuitry includes a parallel-to-serial converter for receiving data output in parallel format from the transmit buffer circuitry using the transmit read clock signal, converting the parallel data into serial format in accordance with a transmit serial clock signal, and providing serial data to a transmit serial interface. The transmit write clock signal and the transmit read clock signal have approximately the same frequency, which is lower than a frequency of the transmit serial clock signal. The transmit counter may be clocked in accordance with the transmit serial clock signal.
In the opposite receive direction, the first clock domain is associated with first processing circuitry, the first clock signal is a receive write clock associated with writing data into receive buffer circuitry, the second clock domain is associated with second processing circuitry, and the second clock signal is a receive read clock signal associated with reading data from the receive buffer circuitry. The counting circuitry includes a receive counter and the first processing circuitry includes a serial-to-parallel converter for outputting in parallel format the receive read clock signal, converting serial data from a serial interface into parallel format in accordance with a receive serial clock signal, and writing parallel data to the receive buffer circuitry in accordance with the receive write clock signal. The receive write clock signal and the receive read clock signal have approximately the same frequency, which is lower than the frequency of the receive serial clock signal. The receive counter is clocked in accordance with the receive serial clock signal.
In one preferred example implementation, the write timing signal relates to writing a known synchronization symbol to the buffer circuitry, and the read timing signal relates to reading the known synchronization symbol from the buffer circuitry. An example synchronization symbol is a K28.5 symbol.
In another example embodiment, the counter circuitry may also include a clock counter configured to receive at a start count input one of the timing signals and to receive at a stop count input a reference timing signal associated with a third clock domain different from the first and second clock domains. The control circuitry performs additional control operations based on the accumulated clock count value.
A particularly advantageous (but still example) application is in the context of a distributed base station that includes a first radio base station node that operates in conjunction with a remote second radio base station node. A base station clock source produces a first reference clock signal. A framer/deframer formats information into frames and transfer frames to/receives frames from a serializer/de-serializer (SERDES) using a second reference clock signal. The SERDES receives the first reference clock signal and generates word clock signals and serial clock signals for transmit and receive paths. The second reference clock and the word clocks have substantially the same frequency but are out of phase. The serial clocks have substantially higher frequency than the second reference clock and the word clocks.
The SERDES includes a transmit buffer for storing a data word from the framer in accordance with the second reference clock signal and for outputting the stored data word in accordance with the transmit word clock signal. The buffering of the data word in the transmit buffer is associated with a transmit buffering delay. A receive buffer stores a received data word in accordance with the receive word clock signal and outputs the stored data word in accordance with the second reference clock signal to the de-framer. The buffering of the data word in the receive buffer is associated with a receive buffering delay.
A transmit counter receives at a start count input a transmit write timing signal associated with the second reference clock signal when the data is written into the transmit buffer. It also receives at a stop count input a receive read timing signal associated with the transmit word clock signal when the data word is read from the transmit buffer. A transmit buffer count value accumulated between receiving the transmit write timing signal and the transmit read timing signal corresponds to the transmit buffer delay. Similarly, a receive counter receives at a start count input a receive write timing signal associated with the receive word clock signal when the data word is written into the receive buffer and receives at a stop count input a receive read timing signal associated with the second reference clock signal when the data word is read from the receive buffer. A receive buffer count value accumulated between receiving the receive write timing signal and the receive read timing signal corresponds to the receive buffer delay. Control circuitry performs a control operation based on one or both of the transmit buffer count value and the receive buffer count value.
These and other features and advantages are further described in connection with the figures and the detailed description.